The long-term objective of this grant application is to understand the immune response mechanisms that are required to successfully control infection with the opportunistic protozoan Toxoplasma gondii, and how ineffective immunity contributes to pathogenesis of disease. The parasite is a major AIDS pathogen and can cause severe disease or death in congenitally infected infants. Toxoplasma induces potent Type 1 cytokine responses that are required in resistance to infection. This proposal focuses on the role of neutrophils in the mouse innate immune response to T. gondii infection. A hallmark characteristic of polymorphonuclear leukocytes (PMN) is their ability to rapidly accumulate in large numbers at foci of infection. Recent work by us and others has revealed an important immunoregulatory role for neutrophils in immune response ignition during microbial infection, and the overall goal of this proposal is to understand in molecular and cellular detail how neutrophils exert this immunoregulatory function. An aim of this proposal is to characterize the role that PMN play in recruitment and activation of dendritic cells. This will be accomplished by examining how parasite-stimulated neutrophils influence dendritic cell activity both in vitro and in vivo. A second aim is to determine the extent to which cytokines and chemokines are present within PMN as preformed pools versus newly synthesized as a result of parasite stimulation, and to determine how exogenous cytokines control release of each cytokine pool. Another goal of this study is to initiate biochemical studies to define the role of MAPK and NFkappaB signaling pathways in control of neutrophil cytokine production during T. gondii stimulation. Finally, Toll-like receptors (TLR) have recently emerged as a major class of pattern recognition receptors involved in innate immune detection of microbial pathogens. The expression pattern of TLR on neutrophils and compared to dendritic cells will be examined. It will also be determined how TLR expression changes during stimulation with T. gondii antigen and during intracellular infection. These studies are expected to clarify how the immune response to Toxoplasma is initiated, and the function of neutrophils in this process. Understanding how immunity is triggered can be expected to lead to more effective means of controlling infection with Toxoplasma and other microbial pathogens.